1. Field of the Invention
The present invention relates to a charging method and a charging circuit for charging a rechargeable secondary battery such as a lithium ion battery, as well as a portable electronic apparatus having the battery or the charging circuit.
2. Description of the Related Art
Secondary batteries, such as lithium ion batteries, which excel in the charging characteristic and are rechargeable are widely used in portable electronic devices including portable telephones, personal handy phone (PHS) terminals, personal digital assistants (PDAs), and personal computers.
Conventionally, in the portable electronic devices using a lithium ion battery as the secondary battery, charging based on constant current control is effected in an initial period of charging, and charging based on constant voltage control is effected in a latter period thereof (e.g., refer to JP-A-8-106926).
This conventional portable electronic device is provided with a constant-current controlling circuit for effecting constant current control and a constant-voltage control circuit for effecting constant voltage control, a DC voltage from an AC adapter being supplied to these circuits. In the initial state of charging, a constant-current controlling transistor of the constant-current controlling circuit is first controlled, and the battery is charged by a constant current. When the battery voltage rises to a predetermined value by this constant current charge, the constant current control is changed over to constant voltage control based on the constant-voltage control circuit, whereupon the battery is charged by a constant voltage by controlling the constant-voltage controlling transistor. When charging is effected up to a rated voltage, charging control is finished.
JP-A-8-106926 is known as a related art.
However, with conventional method of charging control, if the constant current value in constant current control is suppressed to a low level in conformity with the characteristic of the initial charging current of the battery, a long time is required for charging. In addition, if a changeover from constant current charge to constant voltage charge is effected speedily, the charging time can be shortened, but a current greater than a value allowed at that point of time flows due to the difference with the charging voltage of the battery from the point of time of the changeover.
In addition, since the constant-current and constant-voltage controlling transistors function as series regulators, a large heat loss occurs due to the product of a charging current and a differential voltage between the DC voltage from the adapter and the battery voltage. To withstand this heat loss, a control transistor exhibiting greater allowable dissipation must be used.